I. Field of the Invention
The present invention is directed to a device for determining the structural operating characteristics of automobile engines, and, more particularly, to a degree wheel useful in determining the operation of intake and exhaust valves in relation to the top dead center position of their associated piston.
II. Description of the Prior Art
Degree wheels have long been used to determine structural characteristics of automobile engines. Such engines conventionally comprise a crankshaft, one or more pistons operatively connected to the crankshaft by a crank, and a camshaft operatively connected to the crankshaft by means such as gears and/or a timing chain. The camshaft includes cam lobes which cause lifters to open or close the intake and exhaust valves of the cylinders upon rotation of the camshaft. The structural characteristics of the engine to be determined include a determination of the top dead center positions of the pistons, the positions of the intake and exhaust cam lobe center lines relative to the top dead center position, the duration of the valve opening (by determination of the duration of the cam lobe), the overlap of lobe lift, cam phasing and the like. In a conventional four stroke engine, the crankshaft will turn twice per engine cycle, and the camshaft once per engine cycle.
Conventional degree wheels bear degree indicia on one face, disposed at the periphery of the wheel. The degree indicia are conventionally numbered every ten (10) degrees, and are marked with one of two ranges. In one type of degree wheel, degrees are marked by quadrant. From top dead center, extending in either direction, degree readings increase from 0 to 90 degrees, and then decrease back to 0, meeting at bottom dead center on the degree wheel. In another type of degree wheel, the degree markings range from 0.degree. to 180.degree. in each direction, increasing from top dead center to bottom dead center.
In use, either type of degree wheel is rigidly fixed to the crankshaft pulley, with the face of the wheel bearing the degree indicia facing away from the crankshaft. An indicator which registers with the degree indicia is affixed to the engine. The engine is then manually rotated until the desired structural events occur, and corresponding degree readings are obtained.
For example, the location of top dead center for a particular piston is conventionally determined by the travel of the piston to symmetrical points of depth in its cylinder. A depth gauge is used to determine when the symmetrical depths are achieved. A degree reading is obtained for each of these locations, and the midpoint between these two degree readings is the top dead center of piston travel. Bottom dead center of piston travel can be similarly calculated. This method is more accurate than attempting to determine the point of extreme piston travel by direct inspection, because piston travel within the cylinder is at its minimum at top or bottom dead center.
Once the top dead center or bottom dead center positions are determined, the operator can then determine the angular positions where structural events such as the opening and closing of the valves occurs. These positions are determined by measurement of the angular positions of the corresponding cam lobes on the camshaft. The same depth gauge can be employed, abutting either the cam lobe itself, or the upper surface of the lifter which travels on the cam lobe.
Several problems arise in this process. Because many events occur more than either 90.degree. or 180.degree. after top dead center, some sort of notation must be made during the process of measurement, in order to remind the operator of the actual number of degrees traversed. Often it is not convenient to place tape or some other marking on the degree wheel, because of grease and oil present in the work environment. Since the degree wheel is affixed to the crankshaft, top dead center for a particular piston will generally not correspond to a reading of 0.degree. without adjustment of the indicator or pointer. The calculations required to determine the positions of exhaust valve opening and closing, and of the cam lobe durations and center lines, becomes tedious and easily confused. For this reason, evaluation is often made only of the No. 1 cylinder, and this value presumed to be correct for the remaining cylinders. This is, of course, unsatisfactory, since variations between the cam lobes is significant in predicting the performance of the engine. Because of the calculations involved and notations necessary, and the confusion experienced by operators in performing these tasks, it generally takes from one to several days to determine the structural characteristics of all 8 cylinders of an engine.
The way in which degree wheels are presently used also results in the shortening of the useful life of the degree wheel. Damage to the wheel can result because of its rigid affixment to the crankshaft pulley. Conventionally, a degree wheel is trapped between two washers and bolted to the crankshaft, and repeated attachment results in deformation of the wheel. Moreover, if the degree wheel is not firmly attached to the crankshaft, a deleterious change in position of the degree wheel relative to the crankshaft may result. Normally this change in position is viewed as defeating the utility of the degree wheel. The problem of achieving sufficient adjustability of the indicator which registers with the degree markings, while maintaining resistance of that indicator to inadvertent movement during use, has also been noted. Repeated adjustment of a simple rod bolted to the engine may result in its fracture or separation from the engine due to stress.